1. Field of the Invention
This invention relates to electric fuses of relatively sma11 dimension which are used to interrupt relatively small currents in relatively high voltage circuits.
2. Description of the Prior Art
Where electric fuses are used to protect relatively high voltage circuits and are provided with short casings, it is customary to fill those casings with a granular arc-quenching filler material. However, where the amperages of such fuses are small the fill conducts away so much heat from the fusible element that the cross sectional area of the elements must be extremely small to obtain the desired melting characteristics. Such extremely small diameter wires are very difficult to work with when manufacturing such fuses. If no fill is used, the heat conducted from the fusible element will be less and a larger, more manageable wire may be used.
Further, the cross sectional area of the fusible elements of such fuses are so small that if a filler is used it is difficult to obtain uniform heat transfer from the fusible elements to the surrounding arc-quenching filler material. For example, if the arc quenching filler material is a granular material such as quartz sand, there will be minute voids between the grains of the arc-quenching material and the fusible elements, the voids having a lower heat transferring capability than the grains of sand. On the other hand, if the arc-quenching material is a granular material having a finer grain, such as gypsum, the forces which must be used to compact the arc-quenching filler material within the casings of the fuses can displace the fusible elements from their intended positions within the casings, and may even break the fusible elements. Even if a fine grain arc-quenching material such as gypsum, could be compacted around the fusible element without breaking the element, the arc-quenching material could develop cracks or voids when the fuse was handled, such cracks or voids having a lower heat transfer capability than the body of filler material.
Further, regardless of whether a large or fine grain arc-quenching filler material is used, the filling of such small fuse casings is an extremely tedious and delicate operation and the vibration and other handling of the fuse associated with achieving a good void-free fill could often result in breakage of the extremely fine wire-like elements in such low amperage fuses.
Consequently, it is desirable to provide low amperage electric fuses which are capable of protecting relatively high voltage circuits which may be provided with extremely short casings, wire-like elements, and that are devoid of granular arc-quenching filler material. One such electric fuse design is disclosed in U.S. Pat. No. 3,460,086 entitled PROTECTORS FOR ELECTRIC CIRCUITS to A. J. Fister.
The present invention provides an electric fuse which satisfies all of the above requirements and which establishes an impermeable barrier within the fuse casing around the fusible element between the respective end terminals of the fuse to thereby isolate the terminals from one another. As a result, when the fusible element melts, the impermeable barrier prevents the arc plasma from extending between and "feeding" on the end terminals and thus promotes quick extinguishment of the arc. Such a barrier is established without subjecting the fragile wire-like fusible elements to potentially damaging tortuous paths caused by clamping or sandwiching the element between adjacent washers and barriers or the like.